1. Field of the Invention
The invention relates to an apparatus for localizing production errors in a photovoltaic element which is formed substantially by a semiconductor substrate in the form of a wafer, on opposite main surfaces of which are arranged electrical conductors for transporting electrical charge carriers.
2. Description of Related Art
A generally known example of such a photovoltaic element is the crystalline silicon solar cell, which is assembled from layers of semiconducting silicon of different conductivity type, wherein a metallization pattern of a silver-rich material is applied using a screen printing technique to the side adapted to receive incident sunlight (the front side), and wherein the opposite side (the rear side) is wholly covered by a conducting layer (so-called rear side metallization), for instance of aluminium. The metallization pattern is for instance composed of a system of relatively narrow lines (so-called fingers) and a system of relatively wide lines (so-called busbars) connected thereto. During manufacture production errors can occur at numerous locations in such a photovoltaic element, for instance in the fingers, in the contact areas between fingers, busbars or rear side metallization on the one hand and the semiconductor material on the other, or in the semiconductor material itself. These production errors result in a reduced efficiency of the photovoltaic element. If production errors occur systematically, this will of course lead to a high waste during production and accordingly to high costs.
It is an object of the invention to provide an apparatus with which production errors of diverse nature in a photovoltaic element of the type stated in the preamble can be localized, using which the systematic production errors which occur can be identified at an early stage, and waste resulting from occurring systematic errors is limited to a minimum.
It is a further object to provide such an apparatus which is simple to operate, which can be manufactured and serviced at low cost, and which is durable and reliable in use.
These objectives are achieved, and other advantages obtained, with an apparatus of the type specified in the preamble, comprising according to the invention at least one first electrode in electrical contact with a first main surface of said substrate and displaceable over said substrate a second electrode to be arranged in electrical contact with a conductor on the second main surface, displacing means for displacing the first electrode over said first main surface, voltage measuring means for measuring the voltage between the at least one first and the second electrode subject to the position of the first electrode on said first main surface, and adjusting means for adjusting a bias over the electrical conductors on the opposite main surfaces of said substrate.
With an apparatus according to the invention it is possible in simple manner to detect a local voltage change which indicates a production error. Thus detected, systematically occurring voltage changes indicate a systematically occurring production error at the detected position, which error can be further analysed.
By adjusting a bias which in polarity is equal to, and in absolute value is smaller than the voltage at which a photovoltaic element arranged in the dark does become substantially conductive, an apparatus becomes available with which local errors in the semiconductor substrate are detected in simple manner. When such a detection is carried out, the semiconductor wafer in question is arranged in complete darkness. Examples of errors which can be detected with the apparatus applied in this manner are micro-cracks in the semiconductor material, an electrical conductor extending too deeply into the semiconductor material, contamination of the semiconductor material or a short-circuit between the conductors on the two main surfaces of the semiconductor wafer occurring via the edge of the semiconductor wafer.
In one embodiment of an apparatus for localizing production errors, wherein the first main surface is adapted to receive incident light, the apparatus further comprises according to the invention illuminating means for illuminating at least a part of said first main surface.
With this embodiment an apparatus becomes available with which errors can be detected in simple manner in the electrical conductors, in the transitions between electrical conductors and the semiconductor substrate, and in the upper layer of the semiconductor substrate (the emitter). Examples of errors which can be detected with the apparatus applied in this manner are resistances which are too high or cracks in the conductors, poor contacts between conductors and semiconductor substrate and imperfections in the surface of the semiconductor substrate.
In an embodiment wherein the illuminating means are adapted to project a light spot on said first main surface of a substrate on which the arranged electrical conductors extend in substantially parallel manner, the light spot preferably has a diameter which is greater than the distance between two parallel conductors.
In order to localize errors in crystalline silicon solar cells of a known type, wherein the distance between parallel fingers amounts to about 2 to 3 mm, this diameter amounts for instance to about 4 mm, so that there is a uniform illumination by the light spot between the fingers, but the current generated by the illumination is relatively low. As a result voltage gradients on the fingers and busbars are negligibly small, and the apparatus is particularly sensitive to voltage gradients which are the result of variations in the contact resistance between conductors and semiconductor substrate.
In an apparatus according to the invention which is provided with illuminating means, the bias preferably has a value of zero and the adjusting means consist of short-circuiting means for short-circuiting the electrical conductors on the opposite main surfaces of said substrate.
In an advantageous embodiment the first electrode is displaceable over the substrate in two mutually perpendicular directions.
The first electrode comprises for instance a needle directed substantially perpendicularly of the substrate and provided with a contact point, wherein the contact point preferably has a radius which is smaller than the smallest dimension of an electrical conductor on said first main surface.
Said needle is manufactured substantially from a sufficiently hard material, preferably a copper-beryllium alloy, more preferably tungsten. A tungsten needle has the advantage that it breaks easily through an insulating anti-reflection layer on a solar cell and thus makes contact with the emitter layer situated under this anti-reflection layer.